Hills County - CS modification process

[MomandFriend]

DH's divorce papers (10 years ago) indicate that CS for each child ends when they turn 18 or graduate from high school (whichever comes later).   SD18 will be graduating HS in May.   DH lost his job of 13 years in June and has started a new career which pays considerably less.  In fact, he & I agreed that he as long as he makes enough to pay his CS obligation every month, I will cover our monthly expenses.   We did not go for a modification when he lost his job because we knew that CS for SD18 would be going away soon enough - and we just couldn't deal with going through the drama twice!
So when the CS is modified there are two things to take into account - Dh's reduced income and only 1 child.   

Does this process require an attorney?  Or is there a form(s) that we can complete and submit to the court on our own?   Also, how far in advance should we do this in order for it to take effect in June (the month after SD graduates)?

Thank you for your help!